The present invention relates to an anamorphic f.theta. lens system.
Recently, an optical scanning system capable of performing line scanning by deflecting periodically a luminous flux has been employed as a facsimile reception and recording apparatus in which the flux, which travels in a predetermined direction is deflected, in one direction within a predetermined plane by a rotating polyhedric mirror. The luminous flux is caused to converge on a scanning plane by a convergent lens system.
The f.theta. lens system is employed as a convergent lens system in such an optical lens system and serves not only to converge the deflected luminous flux on a scanning surface, but also to move in uniform motion the converged portion of the luminous flux which is deflected at an equal angular velocity. The parallel pencil of rays incident upon the f.theta. lens system with an angle .theta. with respect to the optical axis of the f.theta. lens system, passing through the pupil thereof, is converged upon a point on the scanning surface. The point is separated from the optical axis by a distance proportional to f.theta., where f is the focal distance of the f.theta. lens system. Therefore, in the case where the angle .theta. of the incidence of the pencil of rays on the f.theta. lens system, with respect to the optical axis, is changed at a uniform variation ratio, the spot of the convergent pencil of rays is moved in uniform motion on the scanning surface.
The rotating polyhedric mirror is a regular polygonal pillar or a regular polygonal cylinder whose peripheral surfaces constitute mirrors, and which is rotated about the axis of symmetry of the mirror. A luminous flux incident on the rotating polyhedric mirror from one direction is reflected by one mirror of the rotating polyhedric mirror, and the reflected luminous flux sweeps a predetermined plane as the rotating polyhedric mirror is rotated, and each time the mirror surface for reflecting the luminous flux is changed over during the rotation of the rotating polyhedric mirror, the above-mentioned sweeping is repeated. Of the mirror surfaces of the rotating polyhedric mirror, the mirror which is in a position where the luminous flux enters and which serves to reflect the luminous flux is referred to as a reflecting surface, and a plane which is swept by the reflected luminous flux is referred to as a scanned surface.
When the above-mentioned rotating polyhedric mirror is used, conventionally, there occur the following problems: The angle of each mirror surface of the rotating polyhedric mirror with respect to a rotating axis of the rotating polyhedric mirror differs slightly by a production error, so that the swept surface varies slightly each time the reflecting surface is changed over during the rotation of the rotating polyhedric mirror. In accordance with the variation of the swept surface, the position of the scanning line is slightly changed on the scanning surface and consequently, irregularity of the pitch of the line scanning occurs.
The irregularity of the pitch can be reduced so that the irregularity does not become a substantial problem by sufficiently increasing the production accuracy of the rotating polyhedric mirror. In that case, however, the production cost of the rotating polyhedric mirror becomes quite high.
Alternatively, a method of reducing optically the above-mentioned irregularity of the pitch is known as is disclosed in the Japanese laid-open patent application No. Sho-48-49315, in which a cylindrical lens is disposed in the optical path of a luminous flux and on the side of a light source in view of a rotating polyhedric mirror, and a cylindrical lens or a toroidal lens is disposed on the scanned side. The toroidal lens here means an arc-shaped cylindrical lens.
However, in the case where the irregularity of the pitch is reduced by this method, an experiment shows that the convergency of the convergent pencil of rays is significantly lowered near the starting portion and the end portion of the line scanning when the cylindrical lens is disposed on the above-mentioned scanned side, so that the spot of the convergent pencil of rays becomes blurred. Furthermore, in the case where the toroidal lens is employed, the production cost of the toroidal lens is so high that the cost of the optical scanning apparatus becomes high.